Turbulence

Convective turbulence

Caused by vertical convective currents — rising columns of warm air and the compensating sinking columns around them. For every rising current there's a downward current somewhere; downward currents tend to occur over broader, weaker areas than the concentrated updrafts.

• Visible cue: cumulus clouds. The lumpy, cauliflower appearance of fair-weather cu signals active convection.
• If the air is too dry for clouds to form, convective currents still exist as thermals (dry convection). Glider pilots use them; helicopter pilots feel them.
• Strongest on sunny afternoons over heated terrain, deserts, dark forest canopies, paved surfaces. Weakest at night and over water.

If your morning departure is smooth and your afternoon return is rough, convection is the difference.

Mechanical turbulence

Caused by obstructions in the wind flow — terrain, buildings, structures. Severity depends on wind speed × obstruction size × upwind stability.

• Lee waves: standing waves on the leeward side of a mountain ridge in stable air. AWH Ch. 19 covers the formation conditions; the visual marker is a lenticular cloud or rotor cloud. See the Mountain Flying page for operational technique.
• Wind shear from a thunderstorm can extend several thousand feet above the cell and 20 NM laterally — even in clear air outside the visible cloud.
• Below 2,000 ft AGL, near urban areas, mechanical turbulence from buildings is meaningful for helicopters in slow flight.

Mountain flying literature widely cites 20 kts at ridge level as the threshold above which terrain-induced turbulence becomes a planning problem rather than a comfort issue. Below that, manageable; above, brief and consider alternates.

Reading cloud appearance — the free turbulence forecast

Cloud shape predicts in-cloud turbulence with surprisingly high reliability:

• Smooth, laminar edges and tops → little or no turbulence inside
• Lumpy, non-uniform appearance with visible rolling motion → significant turbulence inside
• Rotor clouds (horizontal-axis rolls just downwind of a ridge) → severe turbulence; avoid by 5+ NM and 2,000+ ft
• Castellanus cu (turreted tops) → unstable mid-level air, building convection likely within hours

Severe or extreme turbulence is reportable as an Urgent PIREP per AIM Chapter 7. File the report — other pilots are about to fly through what you just survived.

Reporting turbulence — intensity ladder

Per AC 00-45H and the AIM 7-1-23 turbulence reporting criteria:

• Light (LGT): momentary slight bumps; passengers may feel slight strain against seat belts.
• Moderate (MOD): changes in altitude/attitude but aircraft remains in positive control. Difficult to walk in cabin. Loose objects move.
• Severe (SEV): large abrupt changes; aircraft momentarily out of control. Loose objects tossed about. Urgent PIREP.
• Extreme (EXTRM): aircraft violently tossed, practically impossible to control. May cause structural damage. Urgent PIREP.

If you feel "moderate" but the autopilot disconnects or you can't hold heading ±10°, you're in severe — upgrade your PIREP language.

Helicopter-specific turbulence behavior

• Helicopter rotor blades are airfoils and control surfaces simultaneously — turbulence that perturbs the disc shows up as both aircraft attitude change and vibration through the controls.
• In strong gusts, reduce airspeed toward maneuvering speed (commonly published as V_NE minus a margin in the RFM); high-speed gust loading is a major retreating-blade-stall trigger — see Hazards: RBS.
• Avoid forced-trim or autopilot fights — many light helicopters have limited yaw stability and turbulence-induced LTE is real. See PPL Aerodynamics: LTE.
• Don't rely on the seatbelt feel as your turbulence sensor — vertical accel registers in your helmet/headset before your hips.